Long chain hydrocarbons present in crude oil are catalytically cracked to increase the yield of valuable short chain hydrocarbons. Unfortunately, the cracking catalysts cannot be completely recovered during the refinery process. As a result, fuels obtained from refinery processes may contain residual catalyst particles (known as catalyst fines or cat fines).
Catalyst fines are predominantly found in fuel oils, and in particular in number 6 fuel oils or heavy fuel oils. Fuel oils are essentially the residue left behind after the useful short chain hydrocarbons have been removed. Fuel oils are therefore the heaviest commercial fuel that can be obtained from crude oil, i.e., heavier than gasoline and naphtha.
It is generally accepted that there are 6 different classes of fuel oil; these are detailed below. The boiling point and carbon chain length of the fuel increases with fuel oil number. Viscosity also increases with number, and the heaviest oil has to be heated to get it to flow.
Number 1 fuel oil is a volatile distillate oil intended for vaporizing pot-type burners. The carbon chain length is typically between 9-16 carbon atoms. Number 2 fuel oil is used either as a home heating oil or as fuel for automobiles, trucks, lorries etc. The carbon chain length is typically between 10-20 carbon atoms. Number 3 fuel oil is a distillate oil for burners requiring low-viscosity fuel. Number 3 fuel oil is now commonly blended with number 2 fuel oil. Number 4 fuel oil is a commercial heating oil for burner installations not equipped with preheaters. The carbon chain length is typically between 12-70 carbon atoms. Number 5 fuel oil is a residual-type industrial heating oil requiring preheating for proper atomization. The carbon chain length is typically between 12-70 carbon atoms.
Number 6 fuel oil is a high-viscosity residual oil requiring preheating to 104-127° C. for ignition. In this context, residual refers to the material remaining after the more valuable fractions of crude oil have been evaporated. The residue usually contains various undesirable impurities, including water, sulphur and catalyst fines. Number 6 fuel oil is also known as residual fuel oil (RFO), heavy fuel oil (HFO), by the Navy specification of Bunker C, or by the Pacific Specification of PS-400. The carbon chain length is typically between 20-70 carbon atoms.
As used herein, the term heavy fuel oil or HFO is referring to number 6 fuel oil or any equivalent thereof. Because of the impurities present in HFO, it is very cheap. In fact, it is the cheapest liquid fuel available. Since it requires heating before use, residual fuel oil cannot be used in road vehicles, as the heating equipment takes up valuable space and makes the vehicle heavier. Heating the HFO is also a delicate procedure, which is inappropriate to do on small, fast moving vehicles. However, marine vessels are typically capable of accommodating the heating equipment necessary to use HFO as a fuel source.
Marine vessels are equipped with settling tanks and purifiers that should reduce the concentration of catalyst fines in the fuel pumped aboard, which according to ISO 8217:2012 should be less than 60 ppm, to beneath the limit recommended by engine manufacturers (15 ppm). Very significant damage can occur to engines if they encounter higher concentrations of cat fines due to, for example, the use of fuel that is not ISO compliant, problems with the fuel purifiers, or rough weather kicking up fines from the bottom of the settling tanks.
Use of HFO as a fuel source is not limited to marine vessels, and historically HFO has been used to power other large engines, such as those found in boilers on steam locomotives, and in large or industrial buildings. HFO is also used to start up boilers in many coal fired power plants.
While there are obvious environmental concerns owing to the presence of residual sulphur in HFO, the presence of catalyst fines is particularly problematic, as catalysts fines are very hard particles that can cause significant damage to engines. In particular, in marine vessels, catalyst fines can cause damage to the cylinder liners. Monitoring for the presence of catalyst fines in HFO is therefore desirable, as it can act as an indicator of the likelihood of damage by catalyst fines from a particular batch of HFO.
Existing methods for detecting catalyst fines are either inadequate, expensive or time consuming. For example, sophisticated equipment has been developed that uses spectroscopy to determine the amount of catalyst fines passing into the engines of the vessel. Examples are the NanoNord Catguard and the Maersk Fluid Technology Sea-Mate M3000 Analyser, which use NMR and XRF spectrometry respectively. However, the high cost (c.a. £50,000) of these machines limits their usefulness. Moreover, such high accuracy methods are overcomplicated; in general, the person in charge of the engine simply needs to know whether there is a significant level of catalyst fines present or not.
It is therefore desirable to find a simple, low cost way to monitor for the presence of catalyst fines in a hydrocarbon sample, particularly heavy fuel oil (HFO).